1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
In recent years, transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Transistors are widely used for electronic devices such as integrated circuits (ICs) and electro-optical devices. In particular, transistors are urgently developed as switching elements of display devices typified by liquid crystal display devices and the like. In an active matrix liquid crystal display device, voltage is applied between a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements.
The range of uses of such an active matrix liquid crystal display device is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix liquid crystal display device have high reliability and that a production method of the active matrix liquid crystal display device offer high yield and reduce production cost.
In active matrix liquid crystal display devices, transistors are mainly used as switching elements. In manufacturing transistors, reduction in the number of photolithography steps is important for cost reduction. For example, one photomask for an eighth generation substrate costs tens of millions of yen, and one photomask for a tenth generation substrate or an eleventh generation substrate costs hundreds of millions of yen. Moreover, even when only one photolithography step is added in the manufacturing process, the number of steps relating to the photolithography step is significantly increased. Therefore, many techniques for reducing the number of photolithography steps have been developed.
Transistors are broadly classified into top-gate transistors, in which a channel formation region is provided below a gate electrode, and bottom-gate transistors, in which a channel formation region is provided above a gate electrode. These transistors are generally manufactured using at least five photomasks.
Many conventional techniques for reducing the number of photolithography steps use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. Using such complicated techniques might cause various problems, thereby leading to reduction in yield. Moreover, there has often been no option but to sacrifice electric characteristics of transistors.
As typical means for reducing the number of photolithography steps in the manufacturing process of a thin film transistor, a technique using a multi-tone mask (also called a half-tone mask or a gray-tone mask) is widely known. As a known technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 is given, for example.
[Reference]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2003-179069